1. Field of the Invention
The present invention is related to an optical low-pass filter having high dust-proofing and infrared-cut properties and an imaging apparatus having the filter.
2. Description of the Related Art
Nowadays, electronic imaging apparatuses which convert an optical image into an electric signal, such as digital still cameras, facsimile machines, scanners, and so on, are widely used. If dust exists in the optical path of a light-receiving surface of an imaging device such as a CCD in an electronic imaging apparatus, the dust will appear in the whole captured image.
For example, as for a digital still single-lens reflex camera with an interchangeable photographing lens, when the photographing lens is removed from the camera body, dust may easily come into the mirror box. In another situation, dust may be generated in the mirror box by the mechanism for controlling the mirror or a diaphragm of a photographing lens. For example, in the case of a facsimile machine or scanner, when a document is sent to a document image reader or the document image reader moves, dust may also be generated. The generated dust may adhere to a light-receiving surface of the CCD or the platen glass. Even when such dust is blown off by a blower, the blown dust remains in the mechanism.
In particular, an optical filter for controlling spatial frequency is located near the imaging device in a digital still camera. A quartz birefringent plate is generally used as the optical filter. Quartz easily collects an electrical charge from vibration and the electrical charge is not easily released because quartz has a piezoelectric effect. Accordingly, dust floating in a camera due to air flow or vibration caused by some operation in the camera may adhere to an optical filter carrying an electrical charge. Accordingly, in order to take a clear photograph, frequent cleaning by an air blower is necessary.
To address this problem, Japanese Unexamined Patent Publication (KOKAI) No. 2001-298640 discloses a digital still camera having a wiper which wipes an outside face of a dust-proofing mechanism. In addition, Japanese Unexamined Patent Publication No. 2002-204379 (U.S. Pub. No. 2004-012714) and Japanese Unexamined Patent Publication No. 2003-319222 (U.S. Pub. Nos. 2003-202114 and 2007-296819) disclose a camera having a holder and a vibrator. The holder has an aperture. A CCD and an optical low-pass filter are mounted in the holder. The aperture is covered and sealed with a dust-proofing member. Dust does not adhere to the CCD and the optical low-pass filter in the holder. In addition, dust adhering to the dust-proofing member is removed by vibration produced by the vibrator. However, the mechanical removal of dust, as disclosed in the above publications has many problems, such as an increase of manufacturing cost, an increase of apparatus weight, an increase of power consumption, and so on.
Furthermore, Japanese Unexamined Patent Publication No. 2006-71851 discloses an optical low-pass filter which is composed of a birefringent and piezoelectric crystal plate. This low-pass filter has a transparent electrically-conductive layer and an anti-reflective layer which are provided on one surface of the filter and an infrared-cut coat or an infrared-and ultraviolet-cut coat which is provided on the other surface. This low-pass filter can prevent dust adherence because static charge is reduced by the transparent electrically-conductive layer.
Furthermore, Japanese Unexamined Patent Publication No. 2006-163275 discloses an optical article having: (1) a substrate which is composed of an optical low-pass filter and an infrared-cut plate, (2) an uncontaminated layer which consists of a material including fluorine and provided on a light-input surface of the substrate, (3) an infrared-cut coat which is provided between the substrate and the uncontaminated layer, and (4) an anti-reflective layer which is provided on a light-output surface of the substrate.
The low-pass filter disclosed in No. 2006-71851 and the optical article in No. 2006-163275 have high infrared property, and can prevent dust adherence, but the dust-proof property is inadequate.